Production of lead chromate



Nov. 18, 1947. R. w. LElsY PRODUCTION OF LEAD CHROIAT Filed Sept.' s, 1943 MW NSK WNQSQQ Y INVENTOR Rez/bem #(ez'qy MMM {f} ECAM ATTORNEYS Patented Nov. 18, 1947 2,430,939 y PRODUCTION OF LEAD OHROMATE Reuben W. Leisy, Pahnerton, Pa., assignor to The New Jersey Zinc Company, New York, N. Y., a corporation of New Jersey Application September 3, 1943, Serial No. 501,061

4 Claims.

This invention is concerned with the production of lead chromate, particularly basic lead chromate, and provides an improved method for the manufacture of lead chromate pigments for use in paints, especially rust protective paints.

It has been proposed heretofore, for example in United States Patent No. 2,044,244, to produce lead chromate by reacting chromic acid with a lead salt in solution. Thus, a reagent capable of forming a soluble lead salt, for example acetic acid or hydrouoric acid, may be added to a slurry of lead oxide, chromic acid being added gradually a little at a time so as to precipitate lead chromate from the water-soluble lead salt formed previously from the lead oxide.

As the result of my investigations, I have discovered that it is possible to form excellent lead Achromate and basic lead chromate pigments by directly reacting chromic acid with lead oxide while the latter is suspended in an aqueous slurry, i. e. without rst dissolving the lead oxide to form a soluble lead salt.

In accordance with my invention, an aqueous slurry of lead oxide (PbO) is treated with an aqueous solution of chromic acid (CrOs) with resulting direct reaction of the two to form (a) lead chromate (PbCrO4), (b) basic lead chromate (Pb2CrO5) or (c) a mixture of the two.

The relative proportions of chromic acid and lead oxide control the proportion of lead chromate and basic lead chromate formed. If one mol of PbO is brought into reaction with one mol of CrOs, the precipitate is lead chromate. aged pulp of a precipitate formed from one mol of CrOa and one mol of PbO in accordance with my invention is acidic in reaction, with a pH of 1.4. Should the ratio of PbO to CrO3 be increased to 3:2, lead chromate (PbCrO4) is formed as in the previous instance, but the aged pulp will be alkaline, with a pH of 8.6. In general, I prefer to operate so as to obtain an alkaline pulp.

I have found that it is particularly desirable to conduct my invention in such fashion as to produce basic lead chromate (PbzCrOs). When two mols of PbO are employed for each mol CrOs, a part of the precipitate consists of lead chromate, the balance being basic lead chromate and some unreacted lead oxide. If, however, the molecular ratio of PbO to CrOa is increased to 3:1, the precipitate is a mixture of Pb2CrO5 with unreacted lead oxide and is free of PbCrO4. The aged pulp obtained in such a reaction is markedly alkaline with a pH of 10.5.

A pigment which yields optimum results in paint under certain conditions is Obtained with The a molecular ratio of PbO to CrOs of 4:1 and, in general, I prefer to employ this ratio. However, excellent results can be obtained with a range of PbO to CrOa ratios of from 3:1 to 6:1. In fact, even higher ratios of PbO to CrOs may be employed. In such cases, however, the product is equivalent to a mixture of lead oxide with the combination of unreacted lead oxide and PbzCrOs that is obtained in the specified range of 3:1 to 6:1.

The presence of PbzCrOs in the pigment produced in accordance with my invention may be shown by the X-ray diffraction pattern of this substance in X-ray diagrams of the pigment.

My invention will be more thoroughly understood in the light of the following detailed description, taken in conjunction with the accompanying flow sheet of apparatus adapted to the practice of the invention.

Referring to the iiow sheet, the equipment comprises a 50 gallon chromic acid solution tank l0 which discharges into a 600 gallon precipitation tank II. This tank is provided with an agitator I2. A pipe I3 on the outlet of the precipitation tank connects through a valved T I4 to a colloid mill I5. The colloid mill can be employed to circulate reacting slurry through a line I6 back into the precipitation tank. The other branch I'I of the T is connected to the inlet of a centrifugal pump I8. This pump can be employed to introduce reacted slurry into the tank I9 of a string filter 20. Pipe 2| serves as an overflow to maintain a constant level in the tank I9. Lead chromate filter cake formed on the string lter is removed to a dryer 22 from whence it passes to a conventional disintegrator 23. The product then goes to a conventional packer 24 and is ready for market.

To take a specific example, lead Oxide in the amount of 899.28 pounds is pulped in 431 gallons of water in the 600-gallon tank by agitation and recirculation of the pulp through the colloid mill for 30 minutes. To the pulp thus formed there is added as rapidly as possible 36 gallons of chromic acid containing 100.72 pounds of CrOa. The pulp to which the chromic acid has been added is agitated vigorously for 2 hours. Thereafter, it is aged for 24 hours while it is kept in suspension by slow agitation. After aging, the solids are removed by ltration on the string lter. The cake thus formed is dried at C., disintegrated and packed.

I claim:

1. The method of producing a lead chromate from chromic acid and lead oxide which comprises subjecting a suspension of solid particles REFERENCES CITED of lead oxide 1n the form of an aqueous slurry to dispersion in a collod mill, and reacting the The following references are of record in the chromic acid directly with the solid particles of file 0f 15h15 patenti zlilecllld Wljle the latter S dispersed in Said 5 UNITED STATES PATENTS ueo ss rr i 1 n 2. Process according to claim 1 in which the bgzrl; Harllgve Junelga ratio of PbO to CrOa employed is such as to yield after aging an alkaline pulp. FOREIGN PATENTS' 3. Process according to claim 1 in which therlO Number C'ountry Date molecular ratio of PbO to CrOx is at least 2:1. 515,995 Great Britain Dec. 20, 1939 4. Process according to claim 1 in which the Y OTHER REFERENCES molecular ratio of PbO to CrOa ranges from 3:1 1.06.1 Mellor, Comprehensive Treatise on Inorganic I & Theoretical Chemistry, vol. XI, 1932; pp. 301 and 302. Longmans, Green & Co., London, New RTEUBEN W. LEISY. YorkToronto. 

